Using summation in the result is not a good idea. The only reason why summation works in this case is because one of the percentages is zero. However, if there were 4 records with 25% individually using summation would add up to 100% in the total line, which would not be correct.

Unless I'm missing something that's not a universally acceptable solution. (A + C) / (B + D) is not equal to (A / B) + (C / D). The former is what should be expected in this situation, but the latter is what you would get if using exception aggregation of summation.